how to make camera moves slower when moving along a curve
With the curve selected;
Select the Object Data tab in the Properties window,
In the Path Animation panel change the Frames value, higher values result in a slower follow speed.




Edit:
This value can be modified in python using the following line;

bpy.data.curves["NurbsPath"].path_duration = X


where NurbsPath is the name of your curve, and X is the value you would like to set.